Amorphous solids examples

Unlike the solid state, the liquid state cannot be characterized by a static description. In a liquid, bonds break and refomi continuously as a fiinction of time. The quantum states in the liquid are similar to those in amorphous solids in the sense that the system is also disordered. The liquid state can be quantified only by considering some ensemble averaging and using statistical measures. For example, consider an elemental liquid. Just as for amorphous solids, one can ask what is the distribution of atoms at a given distance from a reference atom on average, i.e. the radial distribution function or the pair correlation function can also be defined for a liquid. In scattering experiments on liquids, a structure factor is measured. The radial distribution fiinction, g r), is related to the stnicture factor, S q), by... 

As an example, in Fig. 3.11, a schematic two-dimension representation of the structure of cristobalite (a crystalline form of Si02) and of vitreous Si02 is shown. A, B and C represent three cases of double possible equilibrium positions for the atoms of the material in the amorphous state [41]. Atoms can tunnel from one position to another. The thermal excitation of TLS is responsible for the linear contribution to the specific heat of amorphous solids. 

To illustrate this more quantitatively, consider the hypothetical sucrose example discussed by Ahlneck and Zografi [80]. Assuming that all the sorbed water is taken up by the amorphous portion of material, 0.1% total moisture would correspond to approximately 20%, 10%, 4%, and 2% moisture content in the amorphous material, respectively, for 0.5%, 1%, 2.5%, and 5% of amorphous solid. The glass transition temperatures for the amorphous portions... 

At the macroscopic level, a solid is a substance that has both a definite volume and a definite shape. At the microscopic level, solids may be one of two types amorphous or crystalline. Amorphous solids lack extensive ordering of the particles. There is a lack of regularity of the structure. There may be small regions of order separated by large areas of disordered particles. They resemble liquids more than solids in this characteristic. Amorphous solids have no distinct melting point. They simply become softer and softer as the temperature rises. Glass, rubber, and charcoal are examples of amorphous solids. 

The first approach has successfully been applied to the study of amorphous as well as to macroscopically ordered solids. Examples of applications include the determination of backbone geometries in fibrous proteins [4] or the determination of protein-backbone, side-chain, and bound-ligand orientation with respect to the membrane normal in membrane-bound proteins [5-8]. Membranes, bilayers, bicelles, or liposomes are neither solid nor liquid systems but have aspects of both and are sometimes liquid crystalline. In most of these systems, time-independent anisotropic interactions play an important role,... 

The same principles that are valid for the surface of crystalline substances hold for the surface of amorphous solids. Crystals can be of the purely ionic type, e.g., NaF, or of the purely covalent type, e.g., diamond. Most substances, however, are somewhere in between these extremes [even in lithium fluoride, a slight tendency towards bond formation between cations and anions has been shown by precise determinations of the electron density distribution (/)]. Mostly, amorphous solids are found with predominantly covalent bonds. As with liquids, there is usually some close-range ordering of the atoms similar to the ordering in the corresponding crystalline structures. Obviously, this is caused by the tendency of the atoms to retain their normal electron configuration, such as the sp hybridization of silicon in silica. Here, too, transitions from crystalline to amorphous do occur. The microcrystalline forms of carbon which are structurally descended from graphite are an example. 

Solids lacking an ordered, crystalline arrangement are termed amorphous materials, and resemble rigid liquids in structure and properties. Glass (SiO 2) is the classic example of an amorphous solid. Such materials typically soften on heating, rather than showing a sharp melting point. 

CT-VPP-REDOR) or the pulse duration fp (CT-VPD-REDOR) then produces CT-REDOR curves, from which the second moment may be evaluated with distinctively superior accuracy as compared to the values obtained from a parabolic fit to the conventional REDOR data. When restricting the experiment to short dipolar evolution times, the two-spin approximation may be applied for the data analysis, which proves to be especially attractive for amorphous solids, for which the exact spin geometry is unknovm. The data presented on the model compoimds illustrate the various facets of CT-REDOR NMR spectroscopy. First application examples, namely, the evaluation of the heteronuclear Li-Ti dipolar couplings within the garnet structure of Li5La3Nb20i2, the determi-nation of the intemuclear B- P distance in frustrated Lewis pairs, the analysis of Na- F dipolar interaction in fluormica or Na- P... 

Besides structure, X-ray diffraction gives a host of other valuable information. For example, powder diffraction, especially with counter diffractometers, has been widely used for phase identification, quantitative analysis of a mixture of phases, particle size analysis, characterization of physical imperfections (the last two being obtainable from line broadening) and in situ studies of reactions. In the case of amorphous solids, X-ray... 

Stimulated by a variety of commercial applications in fields such as xerography, solar energy conversion, thin-film active devices, and so forth, international interest in this subject area has increased dramatically since these early reports. The absence of long-range order invalidates the use of simplifying concepts such as the Bloch theorem, the counterpart of which has proved elusive for disordered systems. After more than a decade of concentrated research, there remains no example of an amorphous solid for the energy band structure, and the mode of electronic transport is still a subject for continued controversy. 

POEs have also demonstrated a variety of morphological characteristics. For example POE with cyclohexyldimethanol as a monomer unit is present at room temperature as a viscous paste rather than an amorphous solid. Such an embodiment... 

Cyanide bonds to platinum(II) via the carbon atom. The nitrogen atom of the coordinated cyanide ligand reacts with electrophiles forming adducts. Along with examples in Section 52.3, the complex Pt(CN)2- reacts in a similar manner with (jj-Cp)2ZrI2 and (jj5-Cp)2HfI2 in solution to yield polymeric amorphous solids of type [( ) 5-Cp)2ZrPt(CN)4] .274... 

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